In general, railway vehicles are composed of a vehicle body and a bogie that supports the vehicle body, and functional parts such as traction motors, gearboxes, brakes, and the like are mounted to a bogie frame of the bogie. Railway vehicles run on rails on power transmitted from traction motors to wheel sets through gearboxes, and braking is applied by actuating the braking system, which causes the rotation of the wheel sets to be inhibited, for example.
FIG. 1 is a plan view of a conventional bogie frame. FIG. 2 is a cross-sectional view of cross beams of the conventional bogie frame, schematically showing an example of a configuration for mounting a functional part. FIG. 2 is a sectional view, taken along the direction of rails, in which a vehicle body 5 and a center pin 6 of the vehicle body are also clearly shown for ease of understanding of the configuration.
As shown in FIG. 1, the bogie frame includes a pair of left and right side beams 1 disposed along the direction of rails and the cross beams 2 that connect the side beams 1 to each other. The cross beams 2 are constituted by two round pipes having a circular cross section arranged in parallel to each other as shown in FIG. 2, and each of them is welded at both ends thereof to the side beams 1. To ensure integral stiffness of the two round pipe cross beams 2, a pair of upper and lower reinforcing plates 2a and 2b is disposed between the cross beams 2 and welded to them.
The cross beams 2 are provided with mounting brackets 3 for mounting a functional part, each welded to the outer peripheral surfaces of the cross beams 2. The mounting brackets 3 include a mounting bracket 3a for mounting a traction motor, a mounting bracket 3b for mounting a gearbox, and a mounting bracket 3c for mounting a brake. FIG. 2 illustrates a state in which the mounting bracket 3 has been welded to the outside half of the outer peripheral surface of one of the two round pipe cross beams 2, and a traction motor, as a functional part 4, has been attached to the mounting bracket 3. The mounting bracket 3 is formed of a plurality of steel plates welded to each other.
It is to be noted that functional parts such as a traction motor, a gearbox, and a brake impose a heavy load on the bogie frame (cross beams), and therefore it is essential that mounting brackets for mounting a functional part be firm and solid with high fatigue strength. However, a mounting bracket formed of a plurality of steel plates welded to each other has a reduced fatigue strength (according to JIS-E-4207 (Japanese Industrial Standards), for example) at the weld zone where it is welded to the cross beam and at the weld zones of the mounting bracket-forming steel plates where they are welded together, as compared to the base metal regions of the steel plates. Because of this, measures for inhibiting an unwanted decrease in fatigue strength of a mounting bracket are desired.
As a conventional technique that addresses this need, Patent Literature 1 discloses a technique of modifying the shapes of the steel plates that constitute the mounting bracket to reduce stress. Furthermore, Patent Literature 2 discloses a technique of improving the quality of the weld joint by optimizing the welding procedure when fabricating the mounting bracket as well as by modifying the shapes of the steel plates that constitute the mounting bracket.
However, in bogie frames of the conventional type as shown in FIGS. 1 and 2, including the ones disclosed in Patent Literatures 1 and 2, there are many short weld lines and curved weld lines. This is because the mounting bracket 3 is formed of a plurality of steel plates welded to each other, and further, the mounting bracket 3 is welded to the outer peripheral surface of the cross beams 2 which are constituted by round pipes. Consequently, this is likely to reduce the efficiency of the work of welding and increase the difficulty in automatic welding by a welding robot.
Furthermore, as a matter of fact, different types of functional parts having different weights and performance (acting forces, loads, and the like) are used in accordance with individual specifications of railway vehicles. Hence, the positions at which functional parts are to be mounted in the bogie frame (cross beam) cannot be uniformly determined, and thus separate design is necessary for each railway vehicle specifications. In this regard, the conventional techniques address the need by modifying the size and shape of the mounting bracket, i.e., the size and shape of the steel plates constituting the mounting bracket so as to conform to the positions where functional parts which are in conformity with specifications of railway vehicles are mounted. Needless to say, in the design modification, it is necessary to form the mounting bracket into a shape that provides sufficient strength so as to prevent a decrease in fatigue strength of the mounting bracket.
However, with conventional bogie frames, difficulties may arise in mounting a functional part. The difficulties are described in the following.
FIG. 3 is a cross-sectional view of cross beams of the conventional bogie frame, schematically showing an example of the difficulties encountered in mounting a functional part. FIG. 3 shows a sectional view taken along the direction of rails as with FIG. 2.
FIG. 3 shows a case in which a functional part 4 is to be mounted to one of the two round pipe cross beams 2 at a location lower than the top thereof and close thereto, as required by certain specifications of railway vehicles. In this case, there is no space for placement of a mounting bracket and thus the functional part 4 interferes with the cross beam 2. In this case, mounting of the functional part 4 cannot be accomplished by any modifications to the size and shape of the mounting bracket, and therefore the overall dimensional configuration of the bogie frame, including that of the cross beams 2, must be revised. Thus, the conventional bogie frame has a significantly low degree of design flexibility for the positions where functional parts are mounted.